1. Field of the Invention
The present invention relates generally to the fabrication of semiconductor devices and, more particularly, to separately optimizing the front and back side processing of semiconductor wafers.
2. Description of the Related Art
As is well known to those skilled in the art, the fabrication of semiconductor devices involves numerous processing operations. These operations include, for example, impurity implants, gate oxide generation, inter-metal oxide depositions, metallization depositions, photolithography patterning, etching operations, chemical mechanical polishing (CMP), etc. As these operations generate particles and residue, there is a need to clean wafer surfaces, thus removing the contaminants such as adhered particles and adsorbed compounds (e.g., chemicals) from the surfaces of the wafer. It is well known that contaminants should be removed from wafer surfaces, as the existence of such contaminants has a detrimental effect on the performance of the integrated circuit devices.
Initially, a greater emphasis was placed on processing the front side of the wafers as negative consequences associated with presence of contaminants were more pronounced. However, as wafer sizes have increased and feature sizes have decreased, certain shortcomings have been associated with lack of adequate and proper processing of the back side (i.e., non-active side) of wafers. One of such limitations is the deviation of focal plane during the processing of wafers, which is specifically more pronounced during the photolithography-processing step. By way of example, when a wafer is held on a chuck using suction, the presence of contaminants adhered to wafer back side causes the formation of high and low points throughout the wafer surface. As a result, the wafer surface is (locally) tilted, thus creating focal plane deviation. This deviation, although very slight, presents a number of challenges in printing very small features.
In addition to creating focal plane deviation, the contaminant particles have proven to migrate to the wafer front side. The migration may happen during any wet processing steps and/or between the processing tools.
In an attempt to eliminate such drawbacks, double-sided cleaning processing tools have been implemented. One of such double-sided tools is a brush scrubbing tool, which includes a pair of symmetrical brushes. FIGS. 1A and 1B illustrate two types of prior art wafer scrubbers. As shown, FIG. 1A depicts a horizontal wafer scrubber 100 while FIG. 1B shows a vertical wafer scrubber 100xe2x80x2. Both FIGS. 1A and 1B include a pair of brush cores 102a and 102b each having been mounted by a corresponding brush 104a and 104b. As shown, the outer surface of each of the brushes 104a and 104b is covered with a plurality of nodules 105a and 105b, respectively. In both orientations, a wafer 106 is scrubbed as the symmetrical brushes 104a and 104b rotate in a corresponding rotation direction of 108a and 108b. 
As shown, the physical makeup of the brush cores 102a and 102b are identical. In a like manner, the outer surfaces of the brushes 104a and 104b are made out of the same material, thus having the same density and compressibility as well as surface contact areas with the wafer. Therefore, in both configurations of prior art wafer scrubbers, an identical amount of pressure is applied to each of the contact areas as the brushes 104a and 104b are respectively applied to the front and the back side of the wafer 106 with identical forces. Consequently, in prior art wafer scrubbers, the front side and the back side of the wafers are treated identically, even though only the front side of the wafer includes active components.
In view of the foregoing, there is a need for an improved semiconductor processing apparatus and methodology capable of simultaneously processing the front and back side of the wafers differently.
Broadly speaking, the present invention fills these needs by providing an apparatus and methodology capable of asymmetric processing of front and back sides of the wafers. In one embodiment, an asymmetric double-sided brush scrubber is configured to implement asymmetric rollers having different densities, abrasiveness, and/or diameter. In one example, the asymmetric double-sided semiconductor processor is a double-sided brush scrubber configured to clean the front side of the wafer implementing a brush made out of a low density material and the back side of the wafer implementing a brush made out of a high density material. In this manner, each side of the wafer can be treated specific to what is present on that particular side.
It should be appreciated that the present invention can be implemented in numerous ways, including as a process, an apparatus, a system, a device, or a method. Several inventive embodiments of the present invention are described below.
In one embodiment, an asymmetric double-sided substrate scrubber is disclosed. The asymmetric double-sided substrate scrubber includes a first roller and a second roller. The first roller is constructed from a first material having a first density and the second roller is constructed from a second material having a second density. The second density is configured to be greater than the first density. The first roller is configured to be applied onto a first side of a substrate with a first force and the second roller is configured to be applied onto a second side of the substrate with a second force. The second force is configured to be substantially equivalent to the first force.
In one embodiment, as a result of the second density being greater than the first density, the first roller is configured to create a greater contact surface area with the first side of the substrate than the second roller with the second side of the substrate.
In another embodiment, an asymmetric double-sided substrate scrubber is provided. The asymmetric double-sided substrate scrubber includes a first roller having a first diameter and a second roller having a second diameter. The first diameter is configured to be greater than the second diameter. The first roller is configured to be applied onto a first side of a substrate with a first force and the second roller is configured to be applied onto a second side of the substrate with a second force. The second force is configured to be substantially equivalent to the first force. The greater diameter of the first roller is configured to create a greater contact surface area with the first side of the substrate than the second roller with the second side of the substrate.
In yet another embodiment, an asymmetric double-sided substrate scrubber is disclosed. The asymmetric double-sided substrate scrubber includes a pair of first drums and a pair of second drums. The pair of first drums has a first belt-type brush mounted thereon and the pair of second drums has a second belt-type brush mounted thereon. The first belt-type brush is constructed from a first material and is configured to have a first density. The second belt-type brush is constructed from a second material and is configured to have a second density designed to be greater than the first density. The first belt-type brush is configured to be applied onto a first side of a substrate with a first force and the second belt-type brush is configured to be applied onto a second side of the substrate with a second force. The second force is configured to be substantially equivalent to the first force.
In still another embodiment, a method for processing a front side and a back side of a wafer asymmetrically is disclosed. The method includes configuring a first brush for processing the front side of the wafer to have a first compressibility. The method further includes configuring a second brush for processing the back side of the wafer to have a second compressibility. Also included is applying equal force to the front side and the back side of the wafer using the first brush and the second brush. The first brush is configured to achieve a greater compressibility on the front side of the wafer than the second brush on the back side of the wafer.
In yet another embodiment, a method for cleaning a front side and a back side of a wafer asymmetrically is disclosed. The method includes providing a first roller having a material with a first compressibility. Also included is providing a second roller having a material with a second compressibility. The first compressibility is configured to be greater than the second compressibility. The method further includes inserting the wafer such that the first roller having the material with the greater compressibility is closer to the front side of the wafer and the second roller having the material with the lower compressibility is configured to be adjacent to the back side of the wafer. Also included is applying equal force to the front side and the back side of the wafer using the first roller and the second roller. The greater compressibility of the first roller is configured to provide a larger surface area coverage than the second roller with the back side of the wafer. Further included is continuing a scrubbing of the wafer using the first roller and the second roller while applying fluids through one of the brush and a fluid application nozzle. The method also includes discontinuing the scrubbing when the cleaning process removes contaminants on the front side and the back side of the wafer.
The advantages of the present invention are numerous. Most notably, in contrast to the double-sided wafer processors of the prior art, the present invention permits a differential in processing a wafer front side and back side, thus allowing the performance of a greater mechanical action on a selected surface of the wafer. Another advantage is that the embodiments of the present invention can be implemented so as to control the amount of pressure induced on the front side and back side of the wafer despite application of equal forces to the front and back sides of the wafer. Yet another advantage of the present invention is that different magnitude of mechanical action can be performed on the front and back sides of the wafer without substantially bending the wafer or implementing complex systems to control the brush processor.